The invention relates to a mixing apparatus for bulk particulate material, with a multiplicity of outlets distributed over the vertical wall of the cylindrical apparatus communicating inwardly from the edge of the zone containing the particulate material. Descending vertical conduits are provided to communicate between said outlets and a collecting chamber.
It is known that the simultaneous or rapidly changing drawing-off of the material from various levels of the mixing apparatus into a common collecting chamber has a mixing effect since the material is brought together from different phases of temporary variations in composition (DE OS No. 1607773). However, when no special loosening devices are provided, the material flows to these outlets generally not in a uniform manner.
It is also known (DE-AS No. 1507906) to provide centrally in the mixing apparatus a draft flue which has--distributed over its height and its periphery--a multiplicity of outlets, each of them being complemented by a connected pneumatic conveyer chute which protrudes into the mixing apparatus and is designed to convey the pneumatically loosened material to the outlets. This, however, has led to no more satisfactory results. On the one hand it was found that around the draft flue in the area of the air conveyer, chutes operatively connected to the outlets have a narrow annular area formed so the material, due to stronger aeration was more prone to flow and was therefore more easily drawn off, while the greater portion of the mixing apparatus, namely, the areas more distant from the outlets, scarcely participated in the clearing process. On the other hand it was found that the particulate material enters the draft flue almost exclusively through the lowermost outlets, while the upper outlets remain more or less non-participatory. These observations present jointly a picture of poor apparatus clearing and mixing action.
The invention aims at improving the mixing action of the initially mentioned type.
The solution of the invention consists in a structure wherein, in front of each outlet an aeration device is arranged and the corresponding descending vertically disposed tube is deaerated, the collecting chamber being constructed as a homogenizer chamber.
It might have been expected that the aerating devices arranged at the periphery of the mixing apparatus according to the invention would, analogously to the observations on a mixing apparatus with corresponding aerating devices located as a central flue lead to the formation of a more rapidly flowing annular area in the outer zone, which area would exclude the more distant main area of the apparatus contents from the clearing and mixing action. This, however, is surprisingly not the case. This can perhaps be explained by the fact that in the case of the invention the aerating devices are more distant from each other and that therefore their effective ranges do not so readily overlap as in the known device.
Furthermore, one might have been compelled to fear that even in the case of the solution of the invention essentially only the lower material outlets would share in the drawing-off from all outlets essentially simultaneously. This might be traceable, on the one hand, to the pressure conditions in a series of equidistantly positioned vertical tubes. Since the tubes are deaerated, the higher hydrostatic pressure at the lower draw-off devices cannot expand to the entire space of the tubes and thus produce a counterpressure therein which cannot be overcome by the material in the upper draw-off devices. On the contrary, all draw-off devices encounter in these tubes an approximately uniform low counterpressure, so that they can also uniformly participate in the conveyance. On the other hand, the uniform draw-off of the material from all draw-off devices could be connected with the filling and clearing conditions in the tubes. Since these tubes are deaerated, the material in the bottom area of these tubes remains loose, especially in connection with a bottom aeration of these tubes, so that excessive pressure does not originate. On the contrary, the material in these tubes remains fluid so that it can be displaced by the material derived from the lower draw-off devices by means of a suitable choice of the loosening pressure prevailing in these draw-off devices. These draw-off devices participate therefore in the conveyance even when the material level in the tubes should steadily or periodically reach or pass beyond them. Should, by diversified filling and clearing conditions in the fall tubes different draw-off conditions be produced in the individual outlets, such conditions can be easily coped with by a suitable aeration control. By such a control care can also be taken that only limited filling levels in the tubes be produced. For instance, care can be taken that the filling level in the tubes not pass beyond the lowest outlet. The control can instead also be carried out in such a way that the outlets are actuated from the bottom to the top in sequence with increasing filling of the tubes. In operation, the lowest draw-off device can be actuated as the first one after the clearing of the corresponding tubes. As soon as it is reached by the material level or its level is substantially exceeded by that of the material, the next higher draw-off device is actuated, and so forth. When then finally the tubes have reached their maximum filling level, it is emptied into the homogenization chamber, and the process starts anew.
Finally, by the construction of the collecting chamber as a homogenization chamber the result is achieved that the material derived from the various locales of the zone is not only brought together, which would produce short-time variations in composition, but that it is intimately mixed. The combination of the draw-off from various locales on the one hand, and the homogenization on the other hand, therefore permit a mixing effect by which long-time as well as short-time variations in composition are compensated.
Suitably, the aeration devices at the outlets are positioned, at least partly within the zone containing the particulate material so that a loosening effect prevails within the said zone as well.
Furthermore, it is practical that the mentioned vertically disposed tubes be arranged in the zone having the particulate material and each of them be provided with a multiplicity of draw-off devices. This results in a very simple structure without the necessity of breaking through the walls of the mixing apparatus.
According to a further characteristic of the invention, the mentioned tubes are accessible and man-sized, so that each single outlet and the corresponding loosening devices can be inspected and malfunctions can be corrected.
A special characteristic of the invention consists in that the draw-off devices form within a closed space and ascending flow path. The purpose thereby achieved consists in that the material flow can be stopped by shutting-off the aeration flow with practically immediate effect. In a suitable embodiment the ascending flow path is formed by two diaphragm apertures of which the rear diaphragm aperture in the flow path occupies a higher position than the front one, and between the two diaphragm apertures propelling aeration device is provided. As long as the material to be drawn-off is loosened by aeration and behaves therefore similarly to a fluid, it is lifted by the aeration pressure to the level of the second diaphragm aperture. As soon as the aeration is finished, the material flow is stopped.
The possibility of shutting-off the draw-off elements makes an alternate operation thereof possible; for in mixing silos it provides advantageous to draw-off the material not only from different areas of the silo space, but also to carry out this draw-off in periodical variation in order to utilize at each individual draw-off point the advantages of funnel formation.
Advantageously, the homogenization chamber is arranged centrally with respect to the mixing apparatus and connected therewith directly via feed openings which may be arranged more or less centrally at the periphery of the homogenization chamber and/or in the top thereof. Thereby the draw-off at the periphery is supplemented by the outlets explained above by means of one or more centrally arranged openings, so that the draw-off movements and the funnel formation covers all cross-sectional areas of the mixing zone. Such an arrangement is particularly advantageous in mixing apparatus with large cross-sectional dimensions. In these structures, the homogenization chamber is suitably positioned, at least partly, in the mixing zone.